VM: parts of dart2js are never optimized

[rakudrama]

Even when dart2js runs for a long time (batch mode), some methods are not optimized

• Follow the repro instructions for Dart2Js batch mode leaks memory #33708
• profile dart2js after it has been running for 10 minutes

Near the top of the profile you will find something like:

9.21% 9.21% ► 9.21% GCNewSpace
3.46% 3.46% ▼ 3.46% [Stub] OneArgCheckInlineCache
7.94% 1.63%   ► 18.38% Setlet.add
4.66% 0.23%   ▼ 8.09% DominatedUses._compute
4.96% 0.03%     ▼ 100.00% DominatedUses.of

Looking at the code, I see that DominatedUses._compute and Setlet.add are running unoptimized code.
Both account for ~5% of total dart2js compile time, so optimizing the code could net a few percent total gain.

Is there something blocking optimization that we should change?

/cc @mraleph @a-siva

[mraleph]

Thanks for locating this, Stephen! This was the thing I was hunting for the past couple of weeks and I just could not get a grasp on (maybe I was not running dart2js long enough to get a meaningful stack trace).

The underlying bug is misoptimization of monomorphic invocation sites that have type arguments (like cast in Setlet.add). We end up emitting a wrong class check - instead of checking type of the receiver we check type of type arguments - which is always deoptimizes. Which in turn eventually disables optimizations of the method.

This should be fixed by https://dart-review.googlesource.com/c/sdk/+/63380

[mraleph]

This brings around 10% improvement in Dart 2 mode for Dart 2 JS on my measurements.